The need for wire-free broadband channels for data transmission at very high data rates, particularly in the field of mobile satellite communication, is increasing continuously. However, particularly in the aeronautical field, there is a lack of suitable antennas which, in particular, can satisfy the conditions required for mobile use, such as small dimensions and light weight. Furthermore, directional, wire-free data communication with satellites (for example in the Ku or Ka band) is subject to extreme requirements for the transmission characteristic of the antenna systems, since interference of adjacent satellites must be reliably precluded.
In aeronautical applications, the weight and the size of the antenna system are of very major importance, since they reduce the payload of the aircraft, and cause additional operating costs.
The problem is therefore to provide antenna systems which are as small and light as possible and which nevertheless comply with the regulatory requirements for transmission and reception operation when used on mobile carriers.
The regulatory requirements for transmission operation result, for example, from the standards CFR 25.209, CFR 25.222, ITU-R M. 1643 or ETSI EN 302 186. These regulatory regulations are all intended to ensure that no interference with adjacent satellites can occur during directional transmission operation of a mobile satellite antenna. Typical envelopes (envelope curves) of maximum spectral power density are defined for this purpose, as a function of the separation angle to the target satellite. The values specified for a specific separation angle must not be exceeded during transmission operation of the antenna system. This leads to stringent requirements for the angle-dependent antenna characteristic. As one example, FIG. 5a illustrates the requirement from CFR 22.209 for the angle-dependent antenna gain in Ku band in the azimuth direction (tangentially to the Clarke orbit) (bold curve). As the separation angle from the target satellite increases, the antenna gain must decrease sharply. This can be achieved physically only by very homogeneous amplitude and phase configuration of the antenna. Parabolic antennas, which have these characteristics, are therefore typically used. However, antennas such as these are unsuitable for mobile use, in particular on aircraft. Rectangular antenna apertures, or antenna apertures similar to a rectangle, are used to reduce the drag here, with an aspect ratio of the height to width of at most 1:4. Since parabolic mirrors have only very low efficiencies with aspect ratios such as these, antenna arrays are preferably used for applications, for example, on aircraft or motor vehicles.
However, antenna arrays are subject to the known problem of so-called grating lobes. Grating lobes are significant parasitic sidelobes which are created because the beam centers of the antenna elements, which form the antenna array, have to be a certain distance apart from one another, by virtue of the design. At certain beam angles, this leads to positive interference between the antenna elements, and therefore to undesirable emission of electromagnetic power in undesired solid angle ranges. It is evident from the theory of two-dimensional antenna arrays (for example J. D. Kraus and R. J. Marhefka, “Antennas: for all Applications”, 3rd Ed., McGraw-Hill series in electrical engineering, 2002) that significant parasitic grating lobes do not occur only if the beam centers of the antenna array are less than one wavelength apart from one another, at the minimum wavelength that is used.
Since antenna arrays must have a feed network, this results in the practical problem of finding network and antenna array topologies which, on the one hand, satisfy the above condition for the maximum distance between the beam centers, and on the other hand occupy as little physical space as possible. Furthermore, the feed networks must be only minimally dissipative, in order to make it possible to achieve high antenna efficiencies, and therefore minimum antenna sizes.
Furthermore, two independent signal polarizations are typically used in order to increase the data rate for directional satellite communication. The antenna system must therefore be able to process two independent polarizations simultaneously. A high level of polarization separation is required both during transmission operation and during reception operation in order to avoid mixing and therefore efficiency losses. Furthermore, there are strict regulatory requirements for the polarization separation for transmission operation in order to avoid interference with adjacent transponders with orthogonal polarization (cf., for example, CFR 25.209 and 25.222). In the case of antenna arrays, it is therefore on the one hand necessary to ensure that the primary antenna elements have sufficiently good polarization separation, and maintains the polarization sufficiently well, and on the other hand that no undesired mixing of the orthogonal polarizations occurs in the feed networks.
Particularly in the case of aeronautical applications, the required polarization decoupling for linear-polarized signals places very stringent requirements on the antenna system. Since systems such as these are typically mounted on the aircraft fuselage and have a two-axis positioner, the azimuth axis of the antenna aperture always lies on the aircraft plane. The aircraft plane is typically a plane tangential to the Earth's surface. If the aircraft position and the satellite position are now not on the same geographical longitude, then the antenna aperture, when it is pointing at the satellite, is always rotated through a specific angle, which depends on the geographical longitude, with respect to the plane of the Clarke orbit. This so-called geographic skew cannot be compensated for in mobile applications by rotation of the antenna about an axis at right angles to the aperture plane, as is possible with stationary terrestrial antennas. Despite the aspect ratio, which is in principle poor, an aeronautical antenna system must therefore be able to comply with the regulatory requirements even in the presence of a geographic skew, up to a specific rotation angle of typically about ±35°.
This results in the following problems for mobile, in particular aeronautical, satellite antennas, which must be solved simultaneously:                1. minimum possible dimension to comply with the regulatory requirements,        2. maximum antenna efficiency with minimum weight,        3. wide bandwidth in order to cover the reception band and the transmission band (for example, Ku band operation: 10, 7-12, 75 GHz and 13, 75-14, 5 GHz),        4. very good directional characteristic,        5. high polarization separation,        6. compensation for the geographical skew by tracking of the polarization planes of the linear-polarized signals.        
It is known that antennas which are in the form of arrays of horn antenna elements have a very high efficiency. When arrays of horn antenna elements are fed using a network of waveguides, then the attenuation of electromagnetic waves by such networks may be very small. One such array is proposed, for example, in U.S. Pat. No. 5,243,357. However, this is purely a receiving antenna (Column 1, line 10 et seq.). The very high polarization decoupling which is required for operation as a transmitting antenna cannot be achieved with the proposed network of square waveguides. Furthermore, the distance between the antenna elements is comparatively great, by virtue of the design, since the square waveguides must have dimensions in the region of half the wavelength of the frequency being used, in order to guide waves efficiently, and the centers of the antenna elements are therefore far more than one wavelength apart from one another. It is known that this leads to significant sidelobes (so-called grating lobes) in the antenna characteristic. During pure reception operation, these sidelobes are not a problem. However, transmission operation that is permitted in accordance with the regulations is impossible since, for example, CFR 25.209 and CFR 25.222 place very stringent requirements on sidelobe suppression. The polarization separation can be improved by using separate feed networks. For example, U.S. Patent Application Publication No. 2005/0146477 A1 proposes that a dedicated feed network be used in each case for the left-hand circular polarization and the right-hand circular polarization. The antenna elements (in this case aperture crosses) must, however, be fed in a serial form for this purpose. This greatly restricts the usable bandwidth. Typical Ku band operation, for example with a reception band from 10.7 GHz to 12.75 GHz, and a transmission band from 14.0 GHz to 14.5 GHz, is impossible with an arrangement such as this. U.S. Pat. No. 5,568,160, for example, likewise proposes that the distribution network be fed using aperture crosses. However, in this case, primary antenna elements are square horn antenna elements. The feed network breaks down into a network for the horizontal polarization and a network for the vertical polarization. A high level of polarization decoupling is therefore possible. By virtue of the design, the antenna element centers are, however, a comparatively long distance apart from one another, as a result of which parasitic sidelobes occur. The same problem occurs with the arrangements proposed, for example, in U.S. Pat. No. 6,225,960, International Publication No. WO 2006/061865 A1 and GB Patent Application Publication No. 2247990 A. U.S. Pat. No. 6,201,508 proposes that a grid (“crossed septum”; Column 3, line 26) be fitted over each individual horn antenna element, in order to homogenize the aperture configuration. However, by virtue of the design, the beam centers are also far more than one wavelength apart from one another in this case as well, and parasitic sidelobes, which are dependent on the phase correlation, still occur. By virtue of the design, the apparatus also has a considerable height (extent at right angles to the aperture plane), which makes it virtually unusable for mobile, and in particular for aeronautical, applications (“0.37 m” in the Ku band; Column 5, line 15).